Compared to currently used semiconductor (silicon) photoelectric conversion elements, the organic thin-film photoelectric conversion element is more flexible and has a broader range of applications with various shapes and colors. Due to these features, the element is regarded as a highly promising device that can be used in various locations of different conditions. Another attractive point is that the active layer of this element can be efficiently manufactured by a wet process, such as spin-coating or screen-printing; this advantage will ultimately enable the mass production of the device by roll-to-roll processing and significantly reduce the production cost.
However, there are still many problems to be solved before this new device can be put into practical use. For example, the materials for this device are expensive, it must be manufactured under a vacuum or nitrogen atmosphere, and it lacks durability when used under normal atmosphere. These factors resultantly make the device very expensive.
To date, there have been various types of organic photoelectric conversion elements invented. In recent years, one type called the “bulk heterojunction” structure is particularly drawing people's attention due to its high photoelectric conversion efficiency. The element of this type is made of a conductive polymer mixed with a fullerene derivative; the former material corresponds to the p-type semiconductor of the semiconductor photoelectric conversion element and the latter corresponds to the n-type. It is believed that the heterojunction structure, in which the two materials are intricately combined, provides a good level of charge-separation efficiency. Another structure, called the “flat heterojunction cell”, also exhibits similar effects. Although the following description takes the bulk heterojunction structure as an example of the photoelectric conversion layer, the description also applies to the latter structure.
The present cell structure is very simple: a substrate covered with a transparent conductive film (electrode) is spin-coated with a composite of the aforementioned two materials, on which an electrode couple is mounted.
As explainer earlier, the bulk heterojunction structure has good charge-separation efficiency. Unfortunately, the use of organic materials lowers the charge-transfer rate. One effective method for improving its overall photoelectric conversion efficiency is to make the organic layer thinner. However, too thin an organic layer will cause a charge leakage due to a short between the two electrodes and ultimately cause the reverse charge-transport. To avoid this situation, various techniques have been invented thus far.
In a conventionally known type of organic thin-film photoelectric conversion element, the photoelectric conversion efficiency is improved by creating a hole-blocking layer (i.e. a layer that allows electrons to pass through while disallowing the passage of holes) between the metal electrode and the active layer. The hole-blocking layers reported thus far are all made of TiO2 and can be manufactured by the following method: TiO2 is burned on an electrode at a temperature of 450 degrees Celsius within a vacuum chamber from which oxygen and moisture have been removed. This process creates a mesoporous hole-blocking layer of TiO2. Then, a dye layer, which will ultimately serve as the active layer, is applied onto the electrode having the hole-blocking TiO2 layer to obtain the organic thin-film photoelectric conversion element.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-319705
[Non-Patent Document 1] T. Erb et al., Adv. Funct. Mater., 2005, 15, 1193-1196